linux-kernel - Re: [PATCH 2/6] iio: adc: ti-adc128s052: Fix ADC value on BE systems

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Message-ID: <7d871d2d-bb42-43d5-96b0-88f24987d522@gmail.com>
Date: Mon, 31 Mar 2025 15:07:43 +0300
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
 Lars-Peter Clausen <lars@...afoo.de>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Nuno Sa <nuno.sa@...log.com>,
 David Lechner <dlechner@...libre.com>,
 Javier Carrasco <javier.carrasco.cruz@...il.com>, linux-iio@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] iio: adc: ti-adc128s052: Fix ADC value on BE systems

On 31/03/2025 14:11, Jonathan Cameron wrote:
> On Mon, 31 Mar 2025 11:02:55 +0300
> Matti Vaittinen <mazziesaccount@...il.com> wrote:
> 
>> ADCs supported by the ti-adc128s052 driver do return the ADC data in 16
>> bits using big-endian format. The driver does unconditionally swap the
>> bytes. This leads to wrong values being reported to users on big endian
>> systems.
>>
>> Fix this by using the be16_to_cpu() instead of doing unconditional byte
>> swapping.

Appears this was one of the patches I should've never written. Nothing 
went right :) Sorry for the noise. I'll try improving for the v2

> It's not doing unconditional byte swap that I can see. The
> adc->buffer[0] << 8 | adc->buffer[1]
> will work on big or little endian systems as we are explicitly saying
> which byte represents higher bit values in a 16 bit output so on little
> endian it's a byte swap, but on big endian it's a noop (the compiler might
> noticed that and replace this code sequence with an assignment)
> 
> Good cleanup, but not a fix as such unless I'm missing something.

No, you're not missing anything. I am the one who just got confused. I 
am not exactly sure what I was thinking. :rolleyes: This definitely 
isn't a fix. And, as a not a fix needing porting, I may squash this with 
some other patch. I need to take another look at this :)

>> Fixes: 913b86468674 ("iio: adc: Add TI ADC128S052")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Matti Vaittinen <mazziesaccount@...il.com>
>> ---
>> I have no big endian machines on my hands to test this. Problem was
>> spotted by reading the code, which leaves some room for errors.
>> Careful reviewing is appreciated!
>> ---
>>   drivers/iio/adc/ti-adc128s052.c | 13 +++++++------
>>   1 file changed, 7 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/iio/adc/ti-adc128s052.c b/drivers/iio/adc/ti-adc128s052.c
>> index a456ea78462f..d1e31122ea0d 100644
>> --- a/drivers/iio/adc/ti-adc128s052.c
>> +++ b/drivers/iio/adc/ti-adc128s052.c
>> @@ -28,19 +28,20 @@ struct adc128 {
>>   	struct regulator *reg;
>>   	struct mutex lock;
>>   
>> -	u8 buffer[2] __aligned(IIO_DMA_MINALIGN);
>> +	__be16 buffer __aligned(IIO_DMA_MINALIGN);
>>   };
>>   
>>   static int adc128_adc_conversion(struct adc128 *adc, u8 channel)
>>   {
>>   	int ret;
>> +	char *msg = (char *)&adc->buffer;
>>   
>> -	mutex_lock(&adc->lock);
>> +	msg[0] = channel << 3;
>> +	msg[1] = 0;
> 
> Given you are writing shared state why move this out of the lock?

Very Valid Point. I'm not 100% sure what I thought of, probably assumed 
IIO core would serialize the calls. That would've been nasty bug! I 
appreciate your sharp eyes :)

Thanks!

Yours,
	-- Matti